Advances in polymer chemistry and technology over the last few decades have enabled the development of high-performance polymeric fibers. For example, liquid-crystalline polymer solutions of rigid-rod and semi-rigid-rod polymers can be formed into high strength fibers by spinning liquid-crystalline polymer solutions into dope filaments, removing solvent from the dope filaments, washing and drying the fibers; and if desired, further heat treating the dried fibers. One example of high-performance polymeric fibers is para-aramid fiber such as poly(paraphenylene terephthalamide) (“PPD-T” or “PPTA”).
Fiber strength is typically correlated to one or more polymer parameters, including composition, molecular weight, intermolecular interactions, backbone, residual solvent or water, macromolecular orientation, and process history. For example, fiber strength typically increases with polymer length (i.e., molecular weight), polymer orientation, and the presence of strong attractive intermolecular interactions. As high molecular weight rigid-rod polymers are useful for forming polymer solutions (“dopes”) from which fibers can be spun, increasing molecular weight typically results in increased fiber strength.
Fibers derived from 5(6)-amino-2-(p-aminophenyl)benzimidazole, para-phenylenediamine and terephthaloyl dichloride are known in the art. Hydrochloric acid is produced as a by-product of the polymerization reaction. The majority of the fibers made from such copolymers have generally been spun directly from the polymerization solution without further treatment. Such copolymers are the basis for a high strength fibers manufactured in Russia, for example, under the trade names Armos® and Rusar®. See, Russian Patent Application No. 2,045,586. However, the copolymer can be isolated from the polymerization solvent and then redissolved in another solvent, typically sulfuric acid, to spin fibers.
Previously, it was not appreciated that fibers derived from copolymers of 5(6)-amino-2-(p-aminophenyl)benzimidazole, para-phenylenediamine and terephthaloyl dichloride, when spun from sulfuric acid solutions, are exceedingly difficult to neutralize effectively; these fibers retain that sulfuric acid to a much higher degree than other aramid homopolymers. There is a wealth of art teaching that fiber made from sulfuric acid solutions of the aramid homopolymer poly(paraphenylene terephthalamide) can be neutralized/washed quickly and easily because that homopolymer does not have appreciable sites for linkage to the sulfuric acid. Copolymers of 5(6)-amino-2-(p-aminophenyl)benzimidazole, para-phenylenediamine and terephthaloyl dichloride, because of the imidazole functionality, have multiple site that it is believed actually bind the sulfuric acid to the polymer chain. Prior neutralization/washing techniques used for typical homopolymer fiber processing are therefore not adequate for these copolymer fibers.
It is further believed that the copolymer fiber must be sufficiently washed and neutralized to remove essentially all of the sulfuric acid in order to provide a fiber and/or yarn having long-term hydrolytic stability. Therefore, what is needed are new methods to wash and neutralize these copolymer fibers.
Known processes for making copolymer fibers directly from polymerization solution, while producing a good product for use in ballistic and other aramid end-uses, are very expensive with very poor investment economics. As such, there is a need in the art for manufacturing process wherein the copolymer is solutioned in a common solvent, such as sulfuric acid which has both improved economics compared to processes known in the art and provides copolymer fibers having superior long-term physical properties.